In recent years there has been an increased demand for dimmable arc lamp ballasts. Automotive and computer hot cathode fluorescent backlighting require low cost, compact dimmable ballasts with a dimming range of at least 100:1. Dimmable arc discharge ballasts are not new. There have been many patents issued for various dimming methods and circuits. Lamps can be dimmed by varying a current limiting impedance, source frequency, source voltage, or by rapidly switching the lamp on and off using a variable duty cycle to control intensity. Generally, dimming more than a 5:1 range by varying voltage, frequency, or impedance is difficult. A hot cathode fluorescent lamp usually relies on the arc current to heat the cathodes. Below 70% of rated current the cathodes may be insufficiently heated and the lamp may extinguish. Combinations of voltage, frequency, and impedance variation are possible to extend the dimming range of a hot cathode arc lamp, but the resulting circuits are complex and seldom have a dimming range of more than 50:1.
Varying the lamp on/off duty cycle can be used to achieve a wide dimming range. Often referred to as pulse width modulation (PWM), this technique has been used by many to control lamp brightness. U.S. Pat. Nos. 3,863,102, 3,875,458, 4,392,086, 4,392,087, and 4,358,710 teach varying the lamp current by controlling the power line on/off duty cycle. This method results in a narrow lamp dimming range, considerable power line noise, and only works with AC source voltages.
U.S. Pat. No. 4,682,083 operates in a PWM mode where the dimming circuit shorts the lamp out for controlled periods of time. Ballast power is consumed by the dimming circuit resulting in inefficient operation. U.S. Pat. Nos. 4,286,195 and 4,663,570 disclose varying the lamp arc current on/off duty cycle but do not maintain cathode heat resulting in limited dimming range. In addition, U.S. Pat. No. 4,286,195 will only ignite the lamp at the 100 percent intensity level.
U.S. Pat. No. 4,358,716 shorts the ballast's output power state drive circuitry to ground periodically to effect lamp on/off duty cycle control. This patent uses a free-running timer 170 to control the illumination level of the lamp. Typically, this configuration provides a limited duty cycle As a result, illumination level can not be adjusted from full on to full off. While the switch transistors operate at a frequency between 5000 to 250,000 HZ, the lamp filament circuit is operated at 60 HZ (unsynchronized). U.S. Pat. No. 4,087,722 controls the individual widths of the power pulses to the lamp which results in high ballast loss. The other noted PWM patents above control bursts of pulses to the lamp.
It is desirable to have a full on to full off dimming range. The lamp filaments should be constantly powered, especially at low arc current levels where the main arc current is too low to maintain the filaments at thermally emitting temperatures. It is also desirable to preheat the filaments for a period of time prior to applying the arc current to insure that the lamp does not ignite while the coils are cold which would cause cathode coating material to sputter away and reduce lamp life. Further, it is desirable that the lamp starts at any intensity setting including full off. The ballast should not be sensitive to the load such that it would fail in a no lamp load or worn out filament condition. The arc and filament circuits should be frequency synchronized to avoid lamp flicker due to beat frequencies that could result from unsynchronized frequencies. The lamp current waveform should not contain pulses that might exceed the peak rating of the lamp filament coils. Minimal harmonic content in the lamp current waveform is also desired to reduce radio interference caused by the system. The ballast should be able to operate from an AC or DC power source. The ballast should be low cost and integratable to further reduce size and cost.
The above mentioned patents have deficiencies in one or more of the above desired ballast features.